The present invention relates to a new and improved method of, and apparatus for, reducing false alarms in gas warning installations which are caused by spurious gases, and furthermore, serves for determining the composition of a gas mixture, wherein there is used a metal-oxide semiconductor as the gas-sensitive element, this semiconductor being heated to a substantially constant temperature.
It is of utmost importance to provide warning and protection against toxic and explosive gases or chemical vapors, as the case may be, and such warning and protection-installations are presently used in the chemical industry, in traffic facilities, such as garages and tunnels, and in heating installations. There are already available in the market metal-oxide semiconductors which can be used as sensors for this purpose. The semiconductors, when in a heated state, react to reducing gases and vapors as well as H.sub.2 O located in their surroundings by changing their conductivity. This change in conductivity for the semiconductors is beneficially utilized as an indicator of the presence of integral gas concentrations. However, the sensors do not give any information as to which gas components or which chemical vapors they have encountered. Those skilled in the art classify this phenomenon by designating such sensors or detectors as "sensors" which have a "wide-band" response behavior as concerns the gas sensitivity. In contrast thereto, there exists the requirement of determining the course of the concentration of an individual gas or vapor component. This requirement which is present in practical applications has been fulfilled in that, there have been fabricated special gas sensors for a certain gas, for instance selective H.sub.2 S sensors, or for a certain vapor, or that a metal-oxide semiconductor is designed to contain forwardly arranged filters which block undesired gases. What is here disadvantageous is that by providing such an absolute selectivity it is not possible to reliably detect other dangerous gas components or vapor components which simultaneously are located in the surroundings. This will be more fully explained in conjunction with two examples. In a garage there is installed a gas warning installation which selectively responds to carbon monoxide (CO). Gasoline vapors which are formed, for instance, by accidental spilling of gasoline, are therefore not detected. In order to also be able to detect this dangerous gas it is necessary to use wide-band metal-oxide semiconductors which are responsive both to CO and also to the gasoline vapors. By virtue of this compromise there is, however, again reintroduced the problem of false alarms as will be easily discernible from the following considerations. The metal-oxide semiconductor responds with similar sensitivity to both CO and gasoline vapors. Yet, the concentration of noxious carbon monoxide, at which there must be initiated protective measures to protect human beings, amounts to around approximately 50 ppm, whereas gasoline vapors first tend to become explosive at much higher concentrations. The metal-oxide semiconductor, for instance if set to a sensitivity for the detection 50 ppm CO, therefore will erroneously simulate an alarm situation even if there is present a harmless concentration of gasoline vapors. It is for this reason that previously one intentionally dispensed with the detection of the gasoline vapors, so that there were not initiated defensive or protective measures which are especially designed for this purpose.
As the second example mention is made of subterranean communication facilities which are usually run by postal authorities, at least in many European countries, and which are endangered by explosions due to the diffuse intrusion of gases from the public gas pipeline network. Additionally, there are provided in the rooms or areas containing the communication installations accumulators serving for furnishing an emergency current supply and which during their operating times produce hydrogen (H.sub.2). Therefore, to initiate suitable and specific protective measures there exists in such rooms the necessity of being able to distinguish between H.sub.2 as a spurious gas and the explosion-prone gases originating from the gas supply network. Also in this case hydrogen triggers false alarms which heretofore were intentionally tolerated, that is to say, the air circulation and venting installations frequently were unjustifiably turned-on and there was wasted a great deal of electrical energy.
In German Patent Publication No. 2,313,413 and the corresponding U.S. Pat. No. 4,012,692, granted Mar. 15, 1977 there is disclosed a method for the determination of the content of gas components in a gas mixture by means of metal-oxide semiconductors. The metal-oxide semiconductor is specially set to the CO-components in a gas mixture which are encountered in mines. This metal-oxide semiconductor is heated to an upper temperature by a heating wire. During and after the uncontrolled cooling phase down to room or ambient temperature there is accomplished the CO-measurement which is differentiated and integrated at a subsequently connected electronic evaluation circuit. This evaluation only is accomplished at the moment where there is not undertaken any measurement. What is disadvantageous with this prior art equipment is that the described apparatus is only responsive to one gas component and the evaluation operation is separated in time from the measuring operation. The last-mentioned mode of operation leads to delays in giving an alarm.
In order to explain the previous comments somewhat better as concerns the metal-oxide semiconductors reference will be made shortly in this disclosure to FIG. 1 which illustrates the known response behavior of such semiconductor with response profiles, for instance, for two gases.